Radiating release of heat generated by various electronic and electrical devices such as personal computers is a matter of major importance. Thermally conductive compositions and thermally conductive sheets obtained therefrom are used as heat radiating means to allow heat from heat generating parts of electronic and electric devices to escape to heat radiating parts such as heat sinks and metal covers. They are also used for anchoring between electronic parts and heat radiating parts.
Because the thermally conductive sheet is used while being sandwiched between a heat generating part and a heat radiating part, the contacts at the interfaces between the sheet and those parts are important in view of the thermal conductivity. That is, when the contacts at the interfaces are not good, the thermal resistance between the parts and the sheet increases and thermal conductivity becomes lower. Accordingly, the heat conductive sheet must be contacted with a heat generating part and a heat radiating part while the sheet is conforming to fine irregularities present on the surfaces of the parts. The thermally conductive sheet is therefore required to have flexibility and adhesion to the heat generating part and the heat radiating part. However, a thermally conductive sheet with high adhesion generally shows poor handling and is difficult to be released after service.
Japanese Unexamined Patent Publication (Kokai) No. 2000-336279 discloses a thermally conductive composition and a thermally conductive sheet in which wax such as paraffin wax or softening agents, tackifiers and thermally conductive, fillers are added to a thermoplastic resin. Addition of wax to the composition improves adhesion to a body to which the sheet is applied. However, release of the sheet from the body becomes difficult after service. Oil is specifically used as a softening agent. However, volatilization of the oil decreases the flexibility of the composition and adhesion of the sheet to the body, or may cause contamination of electronic parts.
Japanese National Patent Publication (Kohyo) No. 2000-509209 discloses a thermally conductive composition comprising an acrylic pressure-sensitive adhesive component, an α-olefin thermoplastic component, paraffin wax and thermally conductive fillers. Because wax is used, thermally conductive sheets formed from the composition are difficult to be released after service similarly to the composition described in Patent Publication 336279 mentioned above. Although the composition contains an α-olefin thermoplastic component and an acrylic pressure-sensitive adhesive component, adhesion of the sheets to a body to which the sheets are applied becomes insufficient due to an insignificant decrease in viscosity during heating, and the thermal conductivity is lowered.
Japanese Unexamined Patent Publication (Kokai) No. 2001-89756 discloses thermally intermediating materials comprising polymer components, melting point components such as wax or wax-like compounds and thermally conductive fillers. Thermally conductive sheets formed from these compositions are difficult to be released after service similarly to the composition described in Patent Publication 336279. Moreover, the melting point components and polymer components must be selected while the compatibility of these components are taken into consideration, and therefore a flexibility in selections of materials are restricted.
Specification of U.S. Pat. No. 6,399,209 discloses a thermally conductive pad containing a laminate that comprises a flexible layer containing polysiloxane wax having alkyl group substituents and conductive fillers and an anti-blocking layer. A polyester having a glass transition temperature of 70° C. or higher is used as the anti-blocking layer. As a result, the pad is excellent in releasability after service; however, the thermal resistance at the interface is increased due to the high melt viscosity.
Japanese Unexamined Patent Publication (Kokai) No. 2002-234952 discloses heat-release sheets composed of a composition containing a polymer gel (A), a compound (B) that is solid or paste at room temperature and becomes liquid when heated and a thermally conductive filler (C). Specifically, examples of the compound (B) include silicone oils, wax and α-olefins. For such materials, handling of the sheets during releasing is poor, and the thermal conductivity of the sheets are not good because adhesion of the sheets to bodies to which the sheets are applied is decreased due to volatilization of the materials during service of the sheets; moreover, the materials cause contamination of the bodies.